This invention relates generally to methods for sintering silica gel plates and, more particularly, to sintering methods that avoid warping the plates.
Near net shape processing of silica glass plates ordinarily includes a step of sintering. This sintering step can cause warpage in glass plates that are thin, e.g., plates having a length:thickness ratio greater than about 6:1. During sintering, the edges and corners of the plate experience substantially different stresses as compared to the other portions of the plate, and this non-uniform stress profile, in turn, can cause the plate to curve or warp. (See, e.g., C. J. Brinker and G. W. Scherer, xe2x80x9cSol-Gel Science,xe2x80x9d Academic Press, 1990.)
This curvature or warpage usually is detrimental to the final product, and its elimination requires the use of additional processing steps. One such additional processing step that has been used in the past is flattening the glass plate by hot-pressing the plate at a temperature near its melting point. In the case of fused silica, this melting point is quite high (about 1720xc2x0 C.), and hot-pressing at such high temperatures can be considerably expensive.
Another procedure for eliminating warpage of the flat plate is to place a weight on the plate during the sintering step. One drawback to this procedure is that the gel can crack into pieces instead of shrinking uniformly. Moreover, because the weight is necessarily exposed to the same corrosive atmosphere as is the gel, it must be composed of a material that can withstand that atmosphere. In addition, the placement of a large weight on top of the plate can impede the diffusion of gas into the plate.
It should, therefore, be appreciated that there is a significant need for an improved method for sintering sol gel derived glass plates that avoids warpage and also avoids the processing difficulties mentioned above. The present invention provides such a method.
This invention resides in a method for preventing the warpage of gel plates during sintering. In accordance with the invention, a refractory powder is placed between the gel plate and an underlying support surface during the sintering step. The refractory powder partially fuses to both the gel plate and the support surface, to partially fix, or anchor, the gel plate and prevent it from warping. This anchoring takes place in-situ during the sintering process itself, such that a separate processing step is not required.
Preferably, the gel plate is a thin silica gel plate, and a silica-based powder is placed between the silica gel plate and an underlying support surface during the sintering step. The silica-based powder preferably has a sintering temperature between 1150 and 1450xc2x0 Celsius, and more preferably, it sinters above the sintering temperature of the silica gel plate.
Other features and advantages of the invention should become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawing, which illustrates, by way of example, the principles of the invention.